This invention relates to electric power steering, and more particularly, to automatic hysteresis compensation for electric power steering systems.
Typical vehicle steering systems are constructed in such a way that the steering wheel will tend to return to center when let go by the operator of the vehicle. The center is defined as a straight line of travel. This return-to-center function is in response to road forces acting upon the turned front tires of the moving vehicle that tend to force the turned tires to the neutral (or center) position. However, friction in the steering system can interfere with the return-to-center function of the steering. One known method to prevent this is to reduce the overall steering ratio of the steering system, but this has the effect of increasing steering sensitivity, which is not always desirable.
In electric power steering systems, a known technique for preventing steering system friction from interfering with the return-to-center function is to generate a command, in relation to vehicle speed, that commands an electric power steering actuator to provide a return-to-center position force. The force exerted by the actuator overcomes the steering friction and allows the return-to-center function. In some systems, electric power steering motors are controlled either in a current mode or voltage mode. Current mode control provides certain advantages because the inherent control damping and speed dependability of voltage mode control are eliminated. However the elimination of the control damping that is present in the voltage mode can prevent the system from obtaining stable closed-loop poles. One solution provided is by creating an apparatus that includes a torque sensor coupled to a vehicle steering system for measuring steering wheel torque provided by a vehicle operator and, a steering angle sensor coupled to the steering system for measuring steering wheel angle. Furthermore, the apparatus may also include a controller, responsive to the measured steering wheel angle and vehicle speed, that provides a sum return-to-center command, as well as a motor responsive to the sum return-to-center command, a detailed description of which is described in Kaufmann et al., US 5,668,722, issued Sep. 16, 1997, for an Electric Power Steering Control. However, as the operating conditions such as temperature, wear, etc. change, a change in hysteresis, also known as friction or lash, occurs. This change in hysteresis adversely affects the performance of the steering system because the system is operating with a different hysteresis value from what the controller was designed to accommodate.
Disclosed herein are a method and an apparatus by which the hysteresis of a steering system is measured by the steering system controller and used to compute a value of friction compensation that is optimum for a current system state.
In an exemplary preferred embodiment of the invention, a controller continuously monitors the torque and position of the steering system handwheel and records a value of the torque signal each time the position sensor indicates a zero position. These torque values are then separated into two groups, one group denotes a set of torque values for clockwise handwheel movement, and another group denotes a set of torque values for counter-clockwise handwheel movement. A difference between these two groups of data, when suitably filtered for a predetermined period of time forms a measurement of the current system hysteresis. Thus, an enhancement that facilitates consistent performance of the electric power steering feel may be provided. The enhancement equalizes the differences that may exist among different individual electric power steering systems. These differences occur under circumstances such as temperature, wear and tear, and manufacturing variations in different electric power steering systems. From the foregoing it will be seen that this invention is based upon a measurement of a hysteresis of the electric power steering system during its operation. The measurement is possible because a set of sensors senses parameters such as torque or position of the electric power steering system. The relationship of torque and position parameters can be plotted and a symmetrical hysteresis loop curve may be drawn. It is noted that under practical operational circumstances, the loop curve may not be perfectly symmetrical. However, a symmetrical hysteresis curve may be used to best illustrate the concept of the instant invention.